Conventional gas turbine engines include a fan section driven by a core engine. Combustion gases are discharged from the core engine along a primary airflow path and are exhausted through a core exhaust nozzle. Pressurized fan air is discharged through an annular fan nozzle defined at least partially by a fan nacelle and a core nacelle. A majority of propulsion thrust is provided by the pressurized fan air discharged through the fan nozzle, the remainder of the thrust provided from the combustion gases discharged through the core exhaust nozzle.
Fan nozzles of conventional gas turbine engines have fixed geometry. Fixed geometry fan nozzles are a compromise suitable for take-off and landing conditions as well as for cruise conditions as the requirements for take-off and landing conditions are different from requirements for a cruise condition. Some gas turbine engines have implemented fan variable area nozzles. The fan variable area nozzle provides a smaller fan exit nozzle diameter during cruise conditions and a larger fan exit nozzle diameter during take-off and landing conditions to optimize operation at each condition.
Although low pressure ratio turbofans provide high propulsive efficiency low pressure ratio turbofans may be susceptible to fan stability/flutter at low power and low flight speeds. Fan blade flutter signature and flutter boundary management characteristics may change over the life of the engine thereby complicating compensation of the fan stability/flutter issue.